Pulse repeating system



Jan. 9, 1945. $1. CORY ,36

PULSE REPEATING SYSTEM Filed July I0, 1943 a Sheds-Sheet 1 FIG $512012: -DIALING PULSE:- RE-n/lvc DISCO NNECT FROM CALLIIVG- END FIG. 2

SWITCH HOOK ANSWER/VG rusflss 'i FROM CALLED END FIG. 3

FROM CALLED END wybwrok y S./. CORY ATTORNEY v. E 4 2 N t C| M R m N f h To T, 6 w m .H M e N 3 h 5 W V. 2 a a w 5 e s 3 u Jap. 9, 1945.

Jan/9, 1945. s. 1. CORY PULSE REPEATING SYSTEM 3 Shee ts-Sheet 3 /NVENTOR 4 5. CORY gj cmM/QMm ATTORNEY Filed July 10, 1943 AT Es PATENT QPUIiSEREPEATINGLSYSTEM 1 "Samuel I. "CoryfTowaco, N Q JJ, assignor 1 to "Bell Telephone.LLaboratorieS,Incorporated; New

1YorkfN.Y;, arcorporation'of l\lew York Application .rulyio, 1943,:Ser'ial-No,494,197 aph ds. (ours-s4) The: invention :relates "170 :pulse repeating-systems and: more tparticularlywtov systems i of ithis type xoperatinga on .a 1 halie'duplex :basis.

.It; is :an .1 object of the l invention to provide: a

outgdins si n ls wh n theinipdance balance is good. For this purpose thepolar relaym w connected in any well-known manner, as by hav- 111g; its operating windings directly. included i in pulse repeating asystem with a simplified 'duplex n the two balanced circuits or c'onnected in abridge balance andcapable of eflicient operation: under between the :balancedcircuits, inaccordancewith conditions/of line variations-ana interference. well-known ractice in "telegraph repe ting sy :It is a-more: specific object or the invention to t temsJ The systemis also equipped for the transprovide a system: of this-type which with stand-- mission of busy signals comprising a-series or :ardizedequipment isadapted"for repeatin 'se pulses ,tof a :predetermined frequencywusually lecting and supervising :pulses ior telephone about 80 cycles per minute, which are impressed switchingvzsystems over linesr of greatly differing upon the duplex circuitgbyoperationofthe pole characteristicsand lengths. a i changerl f \The. selective supervisory operations of auto- 'As' is well known it hashitherto been necessary matic telephone switching systemsare'frequently to establish a 'fair balance betweenthe line-cirv performed over extensive line circuits, such as 'cuit and the balancing-network in duplex systems toll: lines the lengthsof which: differ greatlyand of: either type referred toabove, in orderto pre- .which :aresubjectedito:well known disturbances. vent the deceiving relay *from responding to the i andinterferences. Thegselecting pulses, usually outgoing pulses. When"signalsareimpressed on termed the dial pulses,are usually transmitted am unbalanced systemunbalance currentsarise from oneitelephone exchange to another "over which maycperate the receiving relay. Thus in half-duplex fpulse lrepeating *circuits, which the differential duplex circuit the currents in the hithertohavebeen carefully balanced to operate tworoperating windings will be unequal or unsatisfactorilyunderthe"changing operating con balance'cl'andthe stronger current may operate ditions insthe line ircuit. "The reason for du the'relay. 'In the bridgeduplex circuit the potenplexing the pulse circuit is mainly to be foundin tialsapplied to the relaywinding areunequal and the fact that theatra'fiic-requirements include the v i an equalizing ort unbalance current will flow transmission in both directions over the line cirthroughthe windings and may operate the relay.

011% of supervisory signals or pulses. The opera- Inaccordance-with afeature of the invention,

tions are furthercoinplicated by the requirement 30113118 balancing network is simplified and is stand that, in theseries of operations involved in the ardized to supply a fair balance fora large variety settingnp of a connection, the line circuit is orlinecircuits so that it may be manufactured placed: undertwo difierentconditions, which may at comparatively low cost. In accordance with be termedztheidle and the busy conditions. g the invention the system is furthermore arranged Thepulse repeating system is adapted tore so that it will be unnecessary to adjust the balceive dialingtpulses: from acalling station, which ancing networkto compensate for sporadic varimay be a remote telephone subscribers stati n ations in theline circuits during ordinary oper or a local operator sydialing circuit, {and to reating conditionsincluding temperature and leak peat: the pulses ovarian outgoingline-circuit to a age changes. This simplification ofthe balancdistant telephone-exchange;equippedfforauto- 40 mg network is further efiective in reducing'the matic switching:in response to'the dial pulsesfor largeamount of designing work involvedinthe the selection of another-exchange or another subprovision of a large variety of differently dimenscribers station. i t i sionedbalancing networks for the corresponding The @repeatingcircuit includes apole'changing varieties ofline circuitsto-satisfy the present day relayffor applying two,differenttransmittingpodsevere demands for accuracy. tentials tothe line circuit corresponding to the For the purposes just outlined the invention two conditions of the pulses, namely, marking provides-anarrangem ent for preventingfalse 0p and spacing or-currentan=d no"current.1 'I'he erations, which would be due to improper repulsesfromthe pole changin'grelayareimpressed sponses by the receiving relay during thetransupon aduplex:circuit-including the line circuit mission fpIIt OiHgpH resulting from 1111- int-one branchand the balancing 'circuitin the balanced conditions. *By this provision a given othenbranch; t l design ofbalancing 'network may be basedon Apolar receiving-relayis includedin'the duplex certain average or predominant lineconditions circuit for reception of signals incoming over. among the large variety of conditions met with in t e line circuit, this *rlay being unaffected by the plant, Suh a network maythusprovidean O CE i accurate balance for a large number of lines with average characteristics; it would however be less accurate in the balancing of line circuits having more diverging characteristics, without, however,

establishing a circuit condition which would result in false operations due to unbalance conditions caused by line variations or interference.

When the pulse repeating system is idle, the pole changing or sending relay will apply an idle condition to the toll line, which may be assumed to be a spacing or'no current condition, the other end of the line circuit being assumed to apply a similar condition to the line. In response to the seizure of the repeater station by a callin station or an operator, the pole changer applies a busy or marking condition to the toll line. After drawings, in which:

receiving relay in one or the other positionin accordance with alternate line conditions.

The invention will now be described more in detail as to its organization and operation and as applied to specific embodiments. In'the description reference will be made to the attached Figs. 1, 2, and 3 are diagrammatic representations of different typical pulses that may be transmitted in either direction over the line circuit;

the transmission of dialing pulses, the distant 5 station also applies a marking or answering 'con. dition to the line circuit which consequently will operatethe polar receiving relay at the repeating station.

After this condition has been reached, further pulses may be transmitted in either direction over the line circuit. For example, a rering pulse may be transmitted by the sending station in the form of a pulse which will cause the distant exchange to rearing the called line; busy flashes may be transmitted back to the calling line; and switchhook flashes may be transmitted in either direction.

The polar receiving relay at the repeater station will respond to incoming pulses from the line circuit and will respond to the answering condition imposed upon the line circuit from the distant end by operating to marking position. Thus, the receiving relay is arranged to control circuit equipment which, in well-known manner, may be arranged to respond in proper sequence to the traffic signals, such as answering and -busy,"

from the called line. It may furthermore be arranged to function in response to dial pulses from the distant station, when the call is in the opposite direction, for control of the selecting operation of telephone switches at the exchange with which the repeater is immediately associated.

It will, therefore, be seen that in a sense the repeater operates under full-duplex conditions, inasmuch as it will operate with either of two polarities applied to either or both ends of the the invention, holding impulses are caused to flow through the receiving relay during the transmission of outgoing pulses to prevent kick-off of the relay, and for this purpose a holding circuit is connected to one or more windings of the receiving relay for applying holding impulses to the relay only during the passing of outgoing transitions. By this provision any unbalance currents from the line and balancing circuits due to the outgoing pulses will be opposed.

'In accordance with another specific feature of the invention, the holding impulse circuit ineludes inductive means for holding or looking the Fig. 4 is a circuit diagram of a repeater circuit representing a specific application of the invention; and

Figs. 5, 6 and 7 are similar circuit diagrams of a repeater circuit representing other specific applications of the invention and including different modifications of the arrangement shown in Fig. 4. r I

Referring to the drawings, it will be assumed that a subscribers station is connected-to a telephone exchange and desires to make a call to a distant subscribers station at another telephone exchange. The two exchanges are connected together over toll lines.

In accordance with established traffic conditions in the telephone plant, the subscriber first removes his receiver from the hook, thereby establishing a calling condition. Thereafter the subscriber operates his calling dial which transmits the usual groups of dialing pulses. The

distant exchange thenapplies an answering condition to the line circuit and rings the called subscriber, Who answersby lifting his. receiver off the hook, thereby establishing a busy condition.

If the called subscriber should bebusy, a con-' The diagram in Fig. 1 shows the signaling current in the line circuit at the time of a. call. Thus, when the line circuit is seized, the line.

current increases from a no current or spacing condition to a steady marking, current condition. Thereafter the marking current is interruped by the dialing pulses, whereupon the line current remains steady. In the case of a subsequent rering the line current is momentarily interrupted.

Fig. 2 refers to the conditions at the called end of the toll line, which normally is applying a spacing condition to the line circuit. Upon answering, the called exchange transmits a steady line current which may subsequently be interrupted by switchhook flashes.

Fig. 3 shows conditions at the called exchange when a, busy condition exists in which case the steady marking condition is interrupted by spacing conditions.

Referring now particularly to Fig. 4, the circuit is that of a repeater station R connected over a toll line circuit TL to a distant exchange station represented at E. The repeater station may be assumed to be located at an exchange to which a plurality of subscribers are connected, one of which is shown at S.

Subscriber station S may be a conventional station with a switchhook SH and a dial D. The

, patent st tion :iis over :azsubacriberis: linciSL totthernepeatersstationt :fioweven entire :nur-

noses rorltlielinvention tthe .rline :cirouit SLsmay :becconnected {to a I toll :operators ceireuih instead ofiitot'acsubscriberis station, iinmwhich ease zthe switchhook may be replaced by a key andithe WI'he repeater :station R ecomprises (the mole changer or sending rrelay 4110 :connected lainwthe line :circuihSL:to -reeeive pulsesand having contacts for alternately; app yinengrourrd and ;batter to theclinercircuit in sendine the :pulsesr out: going from :the exchange. 51A -:souree ofubusy signal may be connected throughairiusy key At I or any automatic means for :iapplying "the line :circuit :SLlfor operation busy signalltozthe etrelay-Jm winding normally holding :the .1 rlayim its :spaoingmosition; and l. two! operating-windings coopereating to operate the arelay ":to markinehrflhe vt-roleswitches; imrsetting 1,111)" a connection vinrresponse to pulses incoming from a distantlaexchaneezto the rep atert B;

lithe l, contacts of z-relay A00 ;:are :also econnected through resistance445 and the primary winding of transformer a'lwto ground. The secondary winding isrdividedginto 'ztwo parts 5 and y-is 1 con nected; over rectifiers :46 I r and "K462 and "over contacts to'f trelay 4611 to "resistances 465 :and '4 66,

which in turn rare connected :in' "series in the .balancingzbranch ofzxthe duplex circuit. The arrangement :is zsuchfithat impulses induced. .in a 1 either ihaifcofi the secondary winding in' response to :operations (of relay 40.0 will anormall-y be applied :inhthessame idirectiontto the balancing cir-.

cuit rthrousht resistance 4.65. However; whenlre iay itllr is operated the-induced impulses xwillrbe applied fin T the P'IBVGISG direction through resistance i466. T:applie.d': to the balancing. branch will be superi imposed 11113011 :the signal pulses from relay 400 contacts of relay 40.0 :are -.connected; through ran 7 induetancei,athrough thedipper operating winding of relay 450, to the toll line TL andcthrough the lower operating winding to the 5 balancing network BN. ,Thus, lathe operation-of: wrela'y 4'00 in 1 sending outgoing ipulses -:will-not ordinarily affect relay 450', since proportionate currents passtthrough-tthe twovwindings-in:opposing; relation in accordance withtthe;balance-between the line-circuitt-TLyand the networkBN.

The distant exchange E may includerassuitable rresponsivetswitch zoontrol ccircuit-M The station E may also include a repeater :cirouit similar to the: circuit R with a. receiving relay similarzto relay lrand with-transmitting contactsliw ll similar :to those of relay dun. y

W hen-the circuit is in the condition t shown in the; drawing, eroun'dtis appliedlato a both :QHdS of tbeyline TLt and toxthebalancing :networkr-BN during the transmission and thus 1 will counteractzany unbalance currentsebetween-v the line cir- "cult :Thand the :balancing network BN during the ssending of *a :transition, .thereby preventing ftkiok-oif" by relayfllifl. l

The operation of the'circuit'will nowibe described; It will: be assumed thatthe circuit is in therrestcondition, .as shown in; the :drawings.

"When ::the subscriber moves :the receiver ,from v the thookaSH,1.-groundis extended over .thesubscriber stlineasL, through the winding of relay 42M] which rquickly operates. Relay-1:00 applies battery tott'herduplex. circuit andaoverthe linelto Sothat no current will befiowing. When-relay 4110 operates, battery is :connected 1 to the bridge pointy-1? and current news through ne win i over v the linetcircuititomroundzat Ewand through the other Minding through the rbalancing network to ig-round. The 'Windings beingwopposed under this condition, -relay "fiflmwould not respond pr vided thevbalanceiis .good. =With relay M10 applying ,gground Ltorthe duplex circuit and contactsmln at the distant end :ofztth e line 1 opertried -,to apply a battery, the incoming 1 line current will pass-throughtthe unper operating" winding to ground at relay lflllvandzin az-directionito overcome lthelbiasingiwinding and operatelrelay 4 50 to marking. If nowirelay 4110 be voperated to also ap ly battery to :the duplex circuit, practi- Ca ly nocurrent: will flow; inathe llnerand -thro'ugh thetupper operating'windine; :the tteryat relayAfilJ will, however, send current through the lowerroperating winding @on,-re1ay .45.0:inha direc-v tion to keep relay 450 operated 1110 -marking. Thus; in receivingincoming, marking, a and spacing pulses, the two operating windings of "relay 45-11 will be in cooperativeirelation tocoperateathe relay to markin or spacing rrespeetively.

Although the receiving relay 450 is shown in the-drawings and is described hereinasl being connected in a differential duplex ul-circuit; iit :should be understood 1 that in accordance With the invention; this relay maybe connected in :a duplex bridgercircuitnwith similarzadvantages.

Relay 450 injmarking-eondition applies ground to relay 460 andaat the sameztime toa'ci-rcuit dueed by lithe transformer isofshort duration to'be effective: in preventing this operationuduring the distant exchange E "for switch ucontrol circuit "49 l ,-.Re1ay =r49fl ;also.=appliesrbattery to the transoperation of the formerTscausinglan induced :impulse .to fiow through resistance 541615. This impulse :will establish: a -.potential difference across :the resistance 1 which will she "poled win the proper direction to oppose :any fortuitous potential established in therduplex:circuit-due toan unbalance condition andso direotedasto tendto causeqan unbalanced current =to rfiow =:which .would operate relay :450 out ;of ,its :zspacing position. The impulse prothe r sendingcofthespace -to-mark transition.

The "subscribernowl operates the dial, the contacts 'of which produce spacingpulses, as shown in *Fig. :1, by short vinterruptions of the ground connection to the line SL. On the arrival of the first :pulse, relay {i400 :releases and applies round to ithe line circuit and-also deenergizes the {primary winding of transformer '1, thereby inducing an. impulse in the secondary winding in the roppositeidirection-of ithe previous-impulse and,ixtherefore passing through ther'ectifier lEZ from tthe :lett-lrand portion of the secondary winding-to ithe resistance 1:46 5 in the same direotion abeforfe; the potential difference :across resistance465 thus again will oppose an'yipotentials in' thee-duplex. circuit tending tocause relay c5 Qflflfito operate out of spacing.

'At pvthe endldf the flrst dial pulse, relay 400 operatesw to apply :battery :to the duplex circuit andi-toitransformer T; thereby again inducing an equipment "410 which is part :of the'exchange equipment. andzsservesiin-rpart ltoiselectivelyl conimpulse tthrough rectifier 46 l and resistance 7465 for holilingirelay M59 in spacing. During each pulse; series :relay Mill awill repeat-each marking and spacing :transition and :will cause a locking impulseitd be produced by? transformer T through resistanceiwfi. :During'an: interval between pulse seriesirelay toil remains attracted and'during the The impulses from thetransformer relay 460 to switch the impulse circuit from transformer T from resistance 465 to resistance 466. Since the rectifiers it! and 462 remain connected to the same point in the balancing branch and the mid-point of the transformer is switched from the right-hand side to the left-hand side of theresistances relative to the rectifier connection, the .impulses produced by the transformer will traverse the resistance 466 in the opposite direction relative to the impulses previously produced in resistance 455. The potential drop across resistance 466 will thus be in a direction to oppose stray potentials in the duplex circuit poled so as to tend to produce unbalance currents which would cause relay 450 to operate out of its marking position.

The desired connection may now be assumed to have been established and the called subscriber will be rung bythe ringing equipment at the exchange E. When the called subscriber answers, the conversation may be carried on over a talking circuit, not shown in the drawings, but which would be closely associated with the toll line TL, as bybeing included in the same cable quad or by some composite arrangement.

It will now be assumed that some special circumstance arises, as for example, that the distant subscriber hangs up prematurely, necessitating the sending of further pulses from the calling end, for example, a rering pulse, as indicated in Fig. 1. The relay 400 would momentarily interrupt thebattery connection and an impulse would be produced through rectifier 462, resistance 465 and over make contact of relay 460 which, as already explained, will prevent kickoff of relay 4551 under the changed line condition. At the end of-the rering pulse relay 400 returns to apply battery to the line circuit, thereby again causing a holding impulse to flow through resistance 466 in the same direction as before, the impulse in this case passing through rectifier 46!.

In the case of a busy condition existing at the distant end after-all dial pulses were transmitted, a series of marking pulses would be transmitted by the contacts 490 over theline TL, through the windings of relay 450 and network BN to ground, causing relay 450 to operate in unison with the busy pulses, and apply corresponding busy pulses to the circuit 410 for suitable response. The consequent operation of relay 460 will have no efiect.

In the case of a call incoming to repeater station R, relay 450 would be operated to marking in response to the application of battery at the distant exchange E to the line. Subsequent interruptions, as by dial pulses, would cause relay 450 to oscillatejand apply corresponding pulses switch for connecting the subscriber station to the repeater in response to-a call by the subscriber. With the subscriber connected in this manner, switchhook flashes may be transmitted back to station E in the manner already described.-

In case the subscriber S is busy, the connection at switch LF will remain open and the circuit 410 may be arranged to automatically operate the key 4| I for application of busy signal from interrupter 4"). This will cause relay 4B0'to operate in unison with the busy pulses and produce holding impulses through resistance 466, as already described.

On observing the busy signal, the distant calling station at E will hang up and thus will send a disconnect signal by applying steady ground over contacts 490, through the windings of relay,

450, thereby operating the relay back to spacing. The circuit 410 responding to this operation may be arranged to automatically disconnect the interrupter 410 by means of contacts 4| I, where upon the whole system will be restored to idle condition.

The inductance I is inserted between the contacts of relay 400 and the apex P for the purpose of shaping the outgoing pulses in any desired manner.

The purpose of the holding circuit for relay 450 is attained during the transmission of outgoing pulses. Ihus, in accordance with the invention and as already explained above, the balancing network is of simple construction and is designed so that networks of this kind may be used universally with toll lines of different lengths and diiierent characteristics. It is, of

course, within the scope of the invention that a few standardized types of network may be provided, each type to be used with lines having characteristics within comparatively narrow limits and the dilferent types of networks supplementing one another to satisfy the requirements of all types of lines. Thus, any one net- Work will be connected to a particular line circult and may establish only anapproximate bal-. ance, and the unbalance may be so pronounced that the receiving relay would kick-01f during transmission of outgoing pulses.

It is furthermore well known that long line CHCllltS as used in toll service, are subject to sporadic variations in their characteristics which are beyond control. Where a very close balance is necessary, it has been the practice to continuously observe the line conditions and correspondingly vary the balance by the network.

, By the provision of the holding circuit for the receiving relay appreciable leeway is afforded 1n the balancing of this circuit, thereby permitting the use of universal networks "for approximate balancing over wide ranges and also in special cases obviating the need for continual readjustment of the network. Thus, during sendmg of pulses any stray potentials tending to produce unbalance currents in or out of the line circuit or the network due to unbalance conditions would be opposed by the potentials established in the balancing circuit by the holding pulses through the transformer T.

By always applying the neutralizing potentials in a direction corresponding to the position-of the receiving relay, it is insured that this relay will not kick-oft under either of the steadystate conditions in the line circuit.

It should be understood that the invention may readily be adapted for application to a telegraph dial emer winding of the receiving relays.

stepsister rent ,atifiesystemwitjii similaradtantags; wit-ere inst twodiiferentsignal sending" takes place as iiigj-tbhditioriS- imposed by "the remote station. it

Referring ewwmere particularly to the'emrecurrent-illustrate Fig. ofthe drawings; the system is jso'mewh'atsiinilar to that-shown i'n'Fig;

ingiy numbered-in two figures i lir-the clr'cuit of 5 the reeeivingrelay 550" has two" separate 'ho'ldingwindings'and the ho1d= ing current issWitdhed through one or the other winding-by relay 56c,- dependent upon the pgsitiener the-receiving relay; Relays San ana 5 re biased polar relaysyha'ving their operat inning serianyin'eluded 'in the subscriber's circuit: S11. Relay sti normally connects grciintl t the holding circuit to short-circuit tlie iioming: windings of relay 5'5 0? In I res onse w each transition ertiie dial pulses; relay 54 meme tarily opens 'the'= short circ'uit; thereby 4" and-corresponding elements" are" correspondseribercallsirelay ato' eperates -temarking and appiies battery to the line circuit TL. Relay'lill respondersthe 'space-tonrark transition in a the circuit S15 and operates, momentarily closing the holding circuit through resistance 6-B5 and the upper holding windin'g oi'relay 658 over tlie spas;- ihg contactof relay 660 to ground, thereby locliirig itherelay 650" in spacing position during: the

causing fih'oldihg impulse to new in one or the Underiithe normal conditions, as shewninrig. i 5; attholdin'g circuit isiclosed through :resistance' 555, :over the break contactof relay 5'50,- through the upper holdingwinding of'relay 5'5 9; resistance iillwto battery.

Winding: ill i heiuing: relay 550 in spacing: i How ven due to the short cirouiting ground at I relay 5*4 l,-no'icurrent normally: flows in the holding circuit;

: When". relay seo perates te placebattery on 1 the'dtlpleir circuitkrel'ay 5dl operates' to markingand during the=travel time the short-circuit is opened, therebypermitting a holding impulse to p H When current fiowsdn circuit,- the holding 1 Winding will "aid 1 the'biasing flow through the upper holding winding during the sending of the space-to-mark transition.

When relay oineratesbackto spacing, relay 54] again momentarily opens the short-circuit for the upper holding windingfand relay 550 will again be held against spacing during the markto-space transition.

With battery supplied te the line eireiiitat the distant end of line TL, relay 550 will be operated to marking and relay 560 will be energized, thereby connecting the lower holding winding into the holding circuit. by relay 500 relay 54! will follow the operations, during each travel time permitting a holding impulse to flow through the lower holding winding of relay 550 which will overcome the effect of any unbalance current in the operating windings of the relay and lock the relay in marking position during outgoing transitions of either polarity.

The circuit arrangement shown in Fig. 6 is During the sending of pulses resistance 143. v i

When the repeater is seized. by the subscriber space *tb -'mark transition.

Duringa mark to space i transition relay; 64 I. will respond to the decaying-current in resistance fifl ahd again niomentarilyclose the holdingcir cuit through the upper holding winding of relay V I. With battery supplied to the toll lineTL from.

the distant exchange E, relay 650 will operate to-marking and relay 660 will be operatedto marking to transferthe holdingcircuit'tfrom the upper to the lowerholding 'winding ola relay 1 650. During eachoutgoingtransitionlrelay fifl Will C1686 "the holding circuit through the lower holding winding, thereby lockingrelay 650 in marking position. l l

The'circuit arrangement shown in Fig. '7 is similar tofthose described above with the principalqmodification that" the receiving relay has'asingle separate holding winding.

nthis arrangement the dialing circuit SIl from. the subscribers station extends through relay T00;

resistance "3 tobatteryi The holding circuit is connected across the resistance 113" to receive a charging impulse through. condenser .142; full- 3 wayerectifier-bridge RB; and through the-holding wihdingof relay 15Din onejdirection for the other; 1 .0

dependent' upon a the-position of relays- 750* and drawingsthe condenser 142 is discharged over the branch b of the rectifier bridge, the right-hand portion of resistance RI, the break contact of relay 160, the branch d of the rectifier bridge and S, relay I00 operates to apply battery to the line circuitTL and the rising current in the pulse circuit and through resistance 143 causes a charging impulse to pass from the upper positive side of resistance 143,.branch a of the rectifier bridge, through the right-hand portion of resistance RI, break contact of relay 160, branch 0 1 of the rectifier bridge; and through the condenser v 142 to the negative side of resistance 143. Due

similar to those shown in Figs. 4 and 5 and is particularly similar to that inFig. 5 in that relay 650 has two additional holding windings. The arrangement difiers from that in Fig. 5 mainly in the fact that the holding circuit is normally open. The relay 64] is connected through condenser 642 across resistance 643 in the suba scribers line circuit SL and thuswill receive The holding impulse the outgoing space-t0- For the sending of a mark-to-space transition the decaying current through resistance 143 will operating potential across the resistance when a current fiows in the circuit SL.

During a rise of current in resistance 643 conpulse will operate the neutral relay 6. During a decline of current through resistance 643 condenser 642 will discharge and the discharge impulse will temporarily operate relay 6M.

With the circuit in the condition shown in the drawings, relay 64!. is released. When thesubcause the condenser M2 to discharge over the branch b, resistance RI, branch d and resistance 143. The potential drop across resistance RI denser 642 will be charged and the charging imdue to the discharge impulse will be in the same direction as before, thereby causing a holding impulse to fiow through relay 15!] to lock the relay inspacing position.

In answering condition the distant exchange E applies battery to the linecircuit, thereby operating relay to marking which, in turn, operates relay 160. Relay 7B0 now transfers the con- With the circuit in the condition shown in the t denser and rectifier circuit to the left-hand portion of resistance RI, thereby reversing the direction of the holding current through relay 150 to lock the relay in marking position against kickoff by unbalance currents arising during outgoing transitions of either polarity. Thus, a charging current in resistance 143 will set up a potential drop across the left-hand portion of resistance RI. In this case a shunt path may be traced from the mid-point of resistance RI through its right-hand portion, through the holding winding to the other end of resistance RI.

What is claimed is:

1. A two-way signaling system comprising an approximately balanced line circuit normally maintained alternately in marking and spacing steady state conditions from the remote end, receiving relay means having contacts for control of a receiving circuit in response to incoming marking and spacing signal pulses superimposed on said steady state conditions, auxiliary circuit means connected to be responsive to outgoing marking and spacing signal pulses to prevent said control by said relay means contacts in response to unbalance currents due to said outgoing pulses, and switching means connected to be responsive to said steady state conditions for the conditioning of said auxiliary circuit means for the proper prevention of said control under both of said steady state conditions, said auxiliary circuit means comprising a locking circuit connected to produce pulsesthrough said receiving relay means for locking said receiving relay means in alternate positions, said locking circuit including two oppositely poled electromagnetic winding means, and said switching means includ ing relay means connected for switching one or the other of said winding means into said locking circuit for reversing the locking action in accordance with said alternate steady state conditions.

i 2. A two-way signaling system comprising an approximately balanced line circuit normally maintained alternately in marking and spacing steady state conditions from therernote end, receiving relay means having contacts for control of a receiving circuit in response to incoming marking and spacing signal pulses superimposed on said steady state conditions, auxiliary circuit means connected to be responsive to outgoing marking and spacing signal pulses to prevent said control by said relay means contacts in response to unbalance currents due to said outgoing pulses, and switching means connected to be responsive to said steady state conditions for conditioning said auxiliary circuit means for the proper prevention of said control under both of said steady state conditions, said auxiliary circuit means comprising an impulse circuit connected to a winding circuit for said receiving relay means for: locking said receiving relay means in alternate positions in accordance with alternate steady 

